The North Atlantic Oscillation and its relationship with near surface temperature (original) (raw)
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Climatic variability over the North Atlantic
International Geophysics, 2002
This paper summarizes an invited presentation given at the historic "Meteorology at the Millennium" Conference in July 2000, which marked the 150 th anniversary of the Royal Meteorological Society. It begins with a broad review of the North Atlantic Oscillation (NAO) and the mechanisms that might influence its phase and amplitude on decadal and longer time scales. New results are presented which suggest an important role for tropical ocean forcing of the unprecedented trend in the wintertime NAO index over the past several decades. We conclude with a brief discussion of a recent significant change in the pattern of the summertime atmospheric circulation over the North Atlantic.
Past and recent changes in the North Atlantic oscillation
Wiley Interdisciplinary Reviews: Climate Change, 2011
The North Atlantic oscillation (NAO) is under current climate conditions the leading mode of atmospheric circulation variability over the North Atlantic region. While the pattern is present during the entire year, it is most important during winter, explaining a large part of the variability of the large-scale pressure field, being thus largely determinant for the weather conditions over the North Atlantic basin and over Western Europe. In this study, a review of recent literature on the basic understanding of the NAO, its variability on different time scales and driving physical mechanisms is presented. In particular, the observed NAO variations and long-term trends are put into a long term perspective by considering paleo-proxy evidence. A representative number of recently released NAO reconstructions are discussed. While the reconstructions agree reasonably well with observations during the instrumental overlapping period, there is a rather high uncertainty between the different reconstructions for the pre-instrumental period, which leads to partially incoherent results, that is, periods where the NAO reconstructions do not agree even in sign. Finally, we highlight the future need of a broader definition of the NAO, the assessment of the stability of the teleconnection centers over time, the analysis of the relations to other relevant variables like temperature and precipitation, as well as on the relevant processes involved.
Climate Dynamics, 2002
The influence of zonal and meridional flow on surface temperature in the North Atlantic/European region is investigated. The degree to which the North Atlantic Oscillation (NAO) index reflects these two different flow types is considered, as is the relationship between the NAO index and surface temperature. Zonal and meridional circulation indices extending back to the early nineteenth or eighteenth centuries are based on surface pressure observations from the North Atlantic and Europe and on an empirical orthogonal function (EOF) analysis of European surface pressure from 1845-1995. The NAO index appears to integrate aspects of both zonal and meridional flow types. The pattern associated with the NAO index is composed of a quadrupole correlation pattern with surface temperature, showing positive correlations over Europe and the Sargasso Sea and negative correlations over northwest Africa and the Greenland/Labrador Sea region. Analysis indicates that the relationship between the NAO index and temperatures downstream of the Atlantic is associated with zonal flow, whereas the influence of the NAO on temperatures upstream is more closely linked to meridional flow patterns. Running correlations indicate that while there is no obvious link between the NAO index and the secular temperature trend, the second principal component of temperature is closely linked to atmospheric circulation, with a relationship which in winter has remained fairly steady through the twentieth century. Notwithstanding this, there have been changes in the strength of the correlation between tem-perature and circulation. These fluctuations in climatecirculation relationships should be further investigated and addressed in studies of climate change, especially in the calibration of paleoclimatic time series and downscaling models.
On the role of the NAO in the recent northeastern Atlantic Arctic warming
Geophysical Research Letters, 2004
1] Studies such as those by and suggest that the northeastern Atlantic Arctic warmed in the early 1990s and that regional sea level pressure (SLP) variations and the NAO may be responsible. Sea surface temperature changes in Fram Strait and the Barents Sea depend, respectively, on SLP variations over the Barents Sea and Norwegian Sea. Since winter 1972, SLP over the Barents and Norwegian Seas has been unusually low during NAO+ winters. Little pressure field change occurred during NAO-winters or around the Denmark Strait, the normal location of the Icelandic Low. Simultaneously, the NAO+ mode became highly persistent on a month-to-month basis throughout the NAO+ winters and ultimately throughout all seasons during a multiyear episode in which the Arctic reached peak warming. A similar NAO+ persistence episode is shown to have occurred from 1920-1925, during another notable Arctic warming event.
A stratospheric influence on the winter NAO and North Atlantic surface climate
Geophysical Research Letters, 2005
1] The North Atlantic Oscillation (NAO) has a profound effect on winter climate variability around the Atlantic basin. Strengthening of the NAO in recent decades has altered surface climate in these regions at a rate far in excess of global mean warming. However, only weak NAO trends are reproduced in climate simulations of the 20th Century, even with prescribed climate forcings and historical seasurface conditions. Here we show that the unexplained strengthening of the NAO can be fully simulated in a climate model by imposing observed trends in the lower stratosphere. This implies that stratospheric variability needs to be reproduced in models to fully simulate surface climate variations in the North Atlantic sector. Despite having little effect on global mean warming, we show that downward coupling of observed stratospheric circulation changes to the surface can account for the majority of change in regional surface climate over Europe and North America between 1965 and 1995.
Climate Research, 2002
A multivariable analysis of the influence of the North Atlantic Oscillation (NAO) on the climate of the North Atlantic and European sectors is presented using the 40 yr (1958-1997) consistent data set from NCEP. Using high and low NAO index composites, anomaly fields of climate variables are then interpreted based on physical mechanisms associated with the anomalous mean flow (characterised by the surface wind field) and the anomalous eddy activity (characterised by the surface vorticity and the 500 hPa storm track fields). It is shown that NAO-related temperature patterns are mainly controlled by the advection of heat by the anomalous mean flow. However, large asymmetries between minimum and maximum temperatures, and more significantly, between positive and negative phases of NAO imply the importance of a different mechanism, namely, the modulation of short wave and long wave radiation by cloud cover variations associated with the NAO. Furthermore, NAO influence on 2 different precipitation-related variables-precipitation rate and precipitable water-displays different patterns. Precipitable water is shown to be strongly related to the corresponding anomaly fields of temperature while precipitation rate appears to be controlled by the surface vorticity field and associated strength of the tropospheric synoptic activity.
Time-integrated North Atlantic Oscillation as a proxy for climatic change
Natural Science, 2013
The time-integrated yearly values of North Atlantic Oscillation (INAO) are found to be well correlated to the sea surface temperature. The results give the feasibility of using INAO as a good proxy for climate change and contribute to a more complete picture of the full range of variability inherent in the climate system. Moreover, the extrapolation in the future of the well identified 65-year harmonic in INAO suggests a gradual decline in global warming starting from 2005.
The North Atlantic Oscillation as an indicator for greenhouse-gas induced regional climate change
Climate Dynamics, 1999
The time-dependent variability of the North Atlantic Oscillation is examined in an observational data set and several model data sets with greenhousegas-induced external forcings. The index of the North Atlantic Oscillation state is derived from the time series of mean latitudinal position and central pressure of the Icelandic Low and the Azores High considering the synchronous meridional shifting of the two pressure systems. While the North Atlantic Oscillation is characterized by intensive interannual variability, the low-pass "ltered index time series shows a decadal component with a time scale of about 50 y within almost 120 y of observation. Since the late 1960s we observe a positive trend and a transition to a strong positive phase of the phenomenon indicative of a predominantly zonal circulation over the North Atlantic. This trend occurs equally in the observations and all examined model data sets with increasing greenhousegas-concentration and atmosphere-ocean coupling. We "nd statistical evidence that the radiative forcing by increasing CO concentration has a signi"cant in#uence on the simulated variability of the North Atlantic Oscillation on time scales of 60 y and longer, independent of the initial conditions and the model version. The seasonal response is strongest in late summer and winter. The interannual variability of the North Atlantic Oscillation states on time scales less than 10 y decreases synchronously with the positive trend of its decadal-mean state implying a stabilization of its present and future zonal state.
The Influence of Surface Forcings on the North Atlantic Oscillation Regime of Winter 2010/11
Monthly Weather Review
December 2010 was unusual both in the strength of the negative North Atlantic Oscillation (NAO) intense atmospheric blocking and the associated record-breaking low temperatures over much of northern Europe. The negative North Atlantic Oscillation for November–January was predicted in October by 8 out of 11 World Meteorological Organization Global Producing Centres (WMO GPCs) of long-range forecasts. This paper examines whether the unusual strength of the NAO and temperature anomaly signals in early winter 2010 are attributable to slowly varying boundary conditions [El Nino–Southern Oscillation state, North Atlantic sea surface temperature (SST) tripole, Arctic sea ice extent, autumn Eurasian snow cover], and whether these were modeled in the Met Office Global Seasonal Forecasting System version 4 (GloSea4). Results from the real-time forecasts showed that a very robust signal was evident in both the surface pressure fields and temperature fields by the beginning of November. The his...